Foamable polyvinylidene fluoride and methods

ABSTRACT

Foamable and foamed polyvinlyidene fluoride materials prepared by compounding the PVDF with a suitable nucleating agent of discreet insoluble particles, a blowing agent having an activation temperature above the melting point of the PVDF, and a dispersing aid to provide a substantially homogeneous powdered mixture that is then melt compounded with high shear melt blending means and then extruded and quenched to provide the foamable polyvinlyidene fluoride material which can then be extruded as an insulating material on wire at a temperature sufficiently high to activate the blowing agent.

BACKGROUND OF THE INVENTION

This invention relates to foamable polyvinylidene fluoride that is useful in forming insulation for electrical wire as well as cable jacketing for such insulated wires.

Usage of polyvinylidene fluoride (PVDF) and copolymers containing a major proportion of PVDF in wire insulation and cable jacketing has grown recently because the relatively low flammability and smoke generation of PVDF permits insulation in plenum duct-work without the need for a costly conduit covering. Accordingly, overall costs and economics of PVDF plenum wiring cable is thus competitive with less expensive insulation materials such as polyvinyl chloride which require conduit shrouding.

The inherently high dielectric constant for PVDF (8-10) renders it unsuitable normally for applications such as wire insulation in long length telephone cables where a low dielectric constant is essential to minimize cross-talk interference. Typical polyvinyl chloride compounds which are currently used have dielectric constants in the 3 to 3.5 range. Through the practice of the instant invention the dielectric constant of PVDF can be reduced to a comparable level by foaming the PVDF material during the wire coating process. The log of the dielectric constant of the foamed PVDF structure is directly proportional to the volume fraction of PVDF with the remainder of the volume being air spaces provided by the foaming process. A PVDF foam with 55% void content has a dielectric constant of about 3.5 and is therefore suitable for many wire insulation applications, including long length telephone primaries.

BRIEF SUMMARY OF THE INVENTION

The method of the invention relating to production of a foamable PVDF material is defined as a method for producing a foamable polyvinylidene fluoride material suitable for forming foamed electrical wire insulation having a dielectic constant of less than about 5.0, comprising:

(a) blending in an intensive powder blender a mixture of

(i) a polymer selected from the group of polymers consisting essentially of a homopolymer of polyvinylidene fluoride, copolymers or terpolymers containing a major portion of vinylidene fluoride with at least one copolymerizable monomer selected from the group consisting essentially of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene and tetrafluoroethylene and mixtures of the homopolymer, copolymer, and terpolymer;

(ii) 0.05 to 5.0 percent by weight of the polymer of (i) of a nucleating agent of discreet, insoluble particles having a size distribution whose average length in the longest dimension of less than about 5 microns;

(iii) 0.05 to 5.0 percent by weight of the polymer of (i) of a blowing agent having an activation temperature that is above the meltng point of said polymer but below the decomposition temperature of said polymer;

(iv) 0.05 to 5.0 percent by weight of the polymer of (i) of a dispersing aid selected from the group of dispersing aids consisting essentially of dialkyl phthalate, dibutoxyethyl phthalate, diisononyl phthalate, ditridecyl phthalate, dicyclohexyl phthalate, diphenyl alkyl adipate, methyl phthalyl ethyl glycolate, and diethylene glycol monobutyl ether;

to provide a substantially homogeneous powdered mixture;

(b) melt compounding the substantially homogeneous powdered mixture of (a) with high shear melt blending means while maintaining the temperature of the mixture below the activation temperature of the blowing agent to provide a substantially homogeneous melted blend of the mixture of (a); and then

(c) extruding and quenching the melted mixture of (b) to provide the foamable polyvinylidene fluoride material.

It is preferred that the polymer include at least 70 mole percent of polyvinylidene fluoride monomer units. Preferably the average size distribution of the length of the nucleating agent is less than about 2 microns length in the longest dimension. Preferred nucleating agents are calcium carbonate, magnesium oxide, lead oxide, barium oxide, titanium oxide, antimony oxide, magnesium carbonate, lead carbonate, zinc carbonate, barium carbonate, carbon black, graphite, alumina, calcium silicate, molybdenum disulfide, magnesium hydroxide, aluminosilicate, calcium metasilicate, and calcium sulfate. It is preferred that the blowing agent be present in an amount of less than about 3 percent by weight of the polymer and that the agent have an activation temperature greater than about 220° C. The most preferable blowing agent is diisopropyl hydrazodicarboxylate and the preferred dispersing aid is dibutyl phthalate or dioctyl phthalate.

The blowing agent is defined as a compound having the formula ROOC--HN--NH--COOR' wherein at least one of R and R' is selected from the group consisting of secondary and tertiary alkyl groups having from 3 to 5 carbon atoms and the other of R and R' is selected from the group consisting of straight-chain and branched-chain alkyl groups having from 1 to 8 carbon atoms, cycloalkyl radicals having from 5 to 8 carbon atoms, aryl radicals having from 6 to 10 carbon atoms, and alkaryl and aralkyl radicals having from 7 to 10 carbon atoms.

As an alternative, the nucleating agent and the dispersing aid can be pre-blended prior to forming the mixture of (a).

Twin screw compounding means are the preferred means for melt compouding the substantially homogeneous powdered mixture of (a).

After step (c) it is preferred that the foamable polyvinlyidene fluoride material be chopped to form pellets.

The method of the invention to provide foamed material is the method as above defined wherein after step (c) the material is heated under sufficient pressure to prevent foaming to a temperature of at least the activation temperature of the blowing agent and then releasing the pressure to provide foamed material.

The method of the invention for providing an electrical insulating sheath of foamed material on a wire is as above described wherein after step (c) the foamable material is heated to a temperature of at least the activation temperature of the blowing agent under sufficient pressure to prevent foaming in an extruder and the heated material is extruded through an electrical wire coating die to provide an insulated wire having an electrical insulating sheath of foamed material on the wire.

The product of the invention includes the foamable polyvinylidene fluoride material, the foamed polyvinlydene fluoride material and an electrical wire insulated with the foamed material made in accordance with the processes as above stated. It is preferred that the foamed material have a dielectric constant of less than about 4.0 and that the insulated wire have a dielectric strength greater than 2 kilovolts.

The foamable PVDF product of this invention is defined as a foamable polyvinlyidene fluoride material suitable for forming foamed electrical wire insulation having a dielectic constant of less than about 5.0, consisting essentially of a substantially homogenous mixture of

(a) a polymer selected from the group of polymers consisting essentially of a homopolymer of polyvinylidene fluoride, copolymers or terpolymers containing a major portion of vinylidene fluoride with at least one copolymerizable monomer selected from the group consisting essentially of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene and tetrafluoroethylene and mixtures of the homopolymer, copolymer, and terpolymer;

(b) 0.05 to 5.0 percent by weight of the polymer of (a) of a nucleating agent of discreet, insoluble particles having a size distribution whose average length is less than about 5 microns length in the longest dimension;

(c) 0.05 to 5.0 percent by weight of the polymer of (a) of a blowing agent having an activation temperature that is above the melting point of said polymer but below the decomposition temperature of said polymer; and

(d) 0.05 to 5.0 percent by weight of the polymer of (a) of a dispersing aid selected from the group of dispersing aids consisting essentially of dialkyl phthalate, dibutoxyethyl phthalate, diisononyl phthalate, ditridecyl phthalate, dicyclohexyl phthalate, diphenyl alkyl adipate, methyl phthalyl ethyl glycolate, and diethylene glycol monobutyl ether.

It is preferred that the polymer include at least 70 mole percent of polyvinylidene fluoride monomer units and that the nucleating agent average size distribution is of less than about 2 microns length in the longest dimension.

The other preferred materials for the foamable product of the invention are as above defined with respect to the process for preparing the foamable product of the invention.

The foamed product of the invention is defined as a foamed polyvinylidene fluoride material consisting essentially of a polymer selected from the group of polymers consisting essentially of a homopolymer of polyvinylidene fluoride, copolymers or terpolymers containing a major portion of vinylidene fluoride with at least one copolymerizable monomer selected from the group consisting essentially of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene and tetrafluoroethylene and mixtures of the homopolymer, copolymer, and terpolymer; said polymer having from 0.05 to 5.0 percent by weight of the polymer of a nucleating agent of discreet, insoluble particles whose average size distribution is less than about 5 microns in the longest dimension dispersed throughout the polymer; said material having a void content within the range of 35% to 55% with a corresponding density of about 1.15 to 0.80 g/cc.

It is preferred that the average cell size longest dimension of the foamed material of the invention be within the range of about 0.5 to 1 mil. and that the polymer include at least 70 mole percent of polyvinlyidene fluoride monomer units. The nucleating agent of the foamed product preferably has an average size distribution in the longest dimension of less than about 2 microns and is preferably a nucleating agent selected from the group consisting essentially of calcium carbonate, magnesium oxide, lead oxide, barium oxide, titanium oxide, antimony oxide, magnesium carbonate, lead carbonate, zinc carbonate, barium carbonate, carbon black, graphite, alumina, calcium silicate, molybdenum disulfide, magnesium hydroxide, aluminosilicate, calcium metasilicate, and calcium sulfate. It is preferred that the foamed material of the invention have a dielectric strength at a 5 mil. thickness of material that is greater than 2 kilovolts.

The foamed product of the invention also includes a foamed material as above defined in the form of an insulating sheath on an electrical conducting wire.

A product of the invention is also the foamed material as above defined in the form of a cable jacket on a bundle of primary electrically insulated wires.

DETAILED DESCRIPTION OF THE INVENTION

The processing equipment required to practice the invention is conventional in nature and readily available. The ingredients comprising the recipe are first subjected to mixing in powder form in an intensive powder blender. Many different types of adequate powder blenders are commercially available. It is possible to pre-blend the nucleating agent and dispersing aid in the intensive powder blender prior to addition of the remainder of the recipe.

Melt compounding of the substantially homogeneous powder recipe blend is preferably done with a readily available twin screw high shear compounder. Such twin screw extruder means are likewise readily available commercially. Although other high shear melt compounding means are available it has been found that the twin screw configuration achieves a higher level of dispersion of the various ingredients of the recipe in a shorter period of time, while maintaining a melt temperature distribution less than the activation temperature of the blowing agent, than is achieved with other types of high shear melt compounders. After extrusion from the melt compounder at a temperature below the activation temperature of the blowing agent, the extruded material is quenched in a water bath and then preferably chopped to form pellets.

The foamable pellets can then subsequently be utilized in a conventional extruder fitted with an electrical wire insulation forming die means to provide coated electrical wire having an insulating sheath of the foamed PVDF material thereon.

Extrusion equipment requirements for the foamable PVDF pellets are similar to those for unmodified PVDF and no additional tooling or equipment modifications are needed. Extrusion of PVDF requires streamlined flow channels to prevent melt stagnation and subsequent discoloration or decomposition. Typical wire extrusion setup and conditions for the foamable PVDF pellets are shown in Table 1 infra.

Conventional screw designs of 24 L/D, 3-5:1 compression ratio, with sharp or gradual transition zones are suitable. A high metering zone temperature is employed to activate the foaming agent. The metering zone is preferred for the activation because the heat transfer to the melt is more effective and uniform, which leads to a more homogenous foam activation and cell structure in the foamed end product. The crosshead should be designed to minimize abrupt internal pressure drops and prevent premature bubble growth inside the die. Ideally, all foam growth should occur just outside the die to minimize open cell formation. Typically, dies with little or no land lengths are recommended for optimum foam structure. However, dies having moderately straight land lengths are satisfactory, particularly with small die orifices.

Typical of all foam wire operations, the quench rate, line speed, wire preheat, die tip temperature, and extrusion rpm have significant effects on the wall thickness, foam structure, and final properties. The qualitative effects of these variables are listed in Table 2 infra and must be adjusted according to the particular equipment setup and desired end product.

The foam structure, and hence properties, are highly dependent upon the processing conditions. PVDF can be processed to yield foamed structures with integrity at a density as low as 0.3 g/cc compared to 1.76 g/cc for the unfoamed PVDF. However, the dielectric strength decreases with a reduction in density. Cell size also influences the dielectric strength behavior, where a reduction in cell size leads to a higher dielectric strength. For a given insulation thickness, the density and cell size must be adjusted to give a suitable dielectric strength which meets the required product specifications.

Properties for a thin wall (0.007 in.) foamed insulation of 0.80 g/cc are presented in Table 3, infra. The insulation was processed using the equipment and conditions specified in Table 1. The foamed insulation has a dielectric constant of 3.6, comparable to that of PVC primary insulation used in long length telephone cables.

As is typical of foams, the foamed PVDF insulation exhibits improved flexibility as noted by the lower flexural modulus and strength. Tensile, and to a smaller extent, flexural strength decrease with decreasing density. The mechanical, as well as the electrical properties, also depend on the cell size; however, for the range of 0.0001 to 0.010-in. cell size, most of the mechanical properties tend to depend mainly on the density. A cable construction containing 24 pairs of foamed PVDF insulated primaries covered with an unfoamed PVDF jacket passed the flame spread and smoke emission requirements of the modified Steiner Tunnel Test (UL910).

                  TABLE 1                                                          ______________________________________                                         TYPICAL EQUIPMENT AND PROCESS CONDITIONS                                       FOR KYNAR 468 FOAM WIRE INSULATION                                             Extruder:       1", 24 L/D                                                     Screw:          1/3 Feed, 1/3 Transition, 1/3 Metering                                         3.5:1 Compression                                              Screen          40/60/100                                                      Pack:                                                                          Die             0.028"                                                         Orifice:                                                                       Die Straight    0.045"                                                         Land Length:                                                                   Conductor:      24 AWG Solid Copper                                                                               Cross  Forming                              Extruder B1      B2     B3    Gate head   Die                                  ______________________________________                                         Temperature Profile (°C.)                                                        210     220    285   230  230    200                                  Screw RPM:      50                                                             Line Speed:     500 fpm                                                        Die-Water Bath Distance:                                                                       0.5"                                                           ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                         GENERAL EFFECTS OF PROCESS VARIABLES ON                                        CN KYNAR 468 FCAM STRUCTURE                                                                 Responses                                                         Process            Insulation        Wall                                      Variables                                                                               Change    Density   Cell Size                                                                              Thickness                                 ______________________________________                                         Line Speed                                                                              +         +         -       -                                         Die-Water                                                                               +         -         +       +                                         Bath Distance                                                                  Extruder +         0         -       +                                         Speed                        (slight)                                          Wire Preheat                                                                            +         -         +       +                                         Temp.                                                                          Mandrel  +         +         -       -                                         Advance                                                                        (In Die)                                                                       ______________________________________                                          KEY:                                                                           + Increase                                                                     0 No significant change                                                        - Decrease                                                               

                                      TABLE 3                                      __________________________________________________________________________     TYPICAL PROPERTIES OF KYNAR 468 FOAMED WIRE INSULATION                                         Test   Foam PVDF                                                                              Virgin PVDF                                                     Method (KYNAR 468)                                                                            (KYNAR 460)                                     __________________________________________________________________________     Wall Thickness (× 10.sup.-3 in)                                                          --      7       7                                              Density (g/cc)          .80     1.76                                           Void Content (%)        55      0                                              Avg. Cell Size (× 10.sup.-3 in)                                                          100 × mag.                                                                      0.6-1.0 --                                              Tensile Modulus (kpsi)                                                                         ASTM D638                                                                             140     290                                             Tensile Break Strength (kpsi)                                                                  ASTM D638                                                                             1.8     6.3                                             Tensile Break Elongation (%)                                                                   ASTM D638                                                                             50-80   100-400                                         Flexural Modulus (kpsi)                                                                        ASTM D790                                                                             135     290                                             Dielectric Constant 100 Hz                                                                     ASTM D1673                                                                            3.6     8.2                                             Dielectric Strength (V/mil)                                                                    ASTM D149                                                                             560     2230                                            Insulation Resistance                                                                          ASTM D3032                                                                            200-300 850                                             (megohm/1000 ft)                                                               __________________________________________________________________________

EXAMPLE 1

A foamed homopolymer of PVDF is prepared as follows:

    ______________________________________                                         Recipe: Material        % by Wt.                                               ______________________________________                                         Polymer - PVDF homopolymer sold by                                                                     95%                                                    Pennwalt Corporation under the                                                 trademark Kynar ® 461                                                      Nucleating Agent (CaCO.sub.3 in snowflake                                                              1.0                                                    form)                                                                          Dispersing aid (dibutylphthalate)                                                                      3.0                                                    Blowing Agent, diisopropyl                                                                             1.0                                                    hydrazodicarboxylate, sold by Uniroyal,                                        Inc. under trademark Celogen ® HT500                                       (Activation temperature, observed in                                           KYNAR blend 240°-250° C.)                                        ______________________________________                                    

The above recipe is then mixed in an intensive powder blender to provide a substantially homogeneous powder blend.

The mixed recipe is then melt blended in a high shear twin screw extruder at 200 rpm and a 200° C. melt temperature to provide adequate dispersing of the recipe blend ingredients, while maintaining the temperature at a level below the activation temperature of the blowing agent. The melt blended recipe is then extruded, quenched in a water bath, and then chopped to form foamable pellets.

The foamable pellets are then extruded as a sheath onto wire to form insulated wire under the conditions set forth in Example 3, infra. The wall thickness of the foamed sheath is 0.007 in. with a density of 0.80 g/cc and a dielectric strength of greater than 400 v./mil. The void content is 55% and the average cell size (determined at 100× magnification) is 0.0006 to 0.0010 inch. The remainder of the properties are as set forth in Table 3.

EXAMPLE 2

Foamed copolymers were prepared from PVDF copolymers containing a minimum of 70 pph vinylidene fluoride units with hexafluoropropylene and/or tetrafluoroethylene, sold under the respective trademarks Kynar Flex 2800™ and KYNAR 7200 by Pennwalt Corporation. The following conditions were used:

    ______________________________________                                         Recipe:                                                                        ______________________________________                                         KYNAR Flex 2800 or KYNAR 7200                                                                        93.5% by weight                                          Celogen ® HT500 (Trademark of                                                                    2.5                                                      Uniroyal, Inc.)                                                                CaCO.sub.3 (Snowflake)                                                                               1.0                                                      Dibutylphthalate      3.0                                                      ______________________________________                                    

The above ingredients were blended for 30 minutes in a moderate intensity powder blender. The KYNAR 2800 and KYNAR 7200 foamable blends were then compounded and pelletized using a twin screw extruder under the following conditions.

    ______________________________________                                         Barrel Zone                                                                    1     2       3      4    5     6    Die  Screw RPM                            ______________________________________                                         250° F.                                                                       350     390    390  390   390  390  200                                  ______________________________________                                    

The pellets were then mixed with 2% violet concentrate (KYNAR color concentrate Type K-50087 available from Pennwalt Corporation) and processed into foamed wire insulation under the conditions stated below:

    ______________________________________                                         1 Inch Extruder 24/1 L/D                                                                Extruder Zone                                                                                           Cross-                                                                               Forming                                         1     2      3      Gate head  Die                                    ______________________________________                                                  210° C.                                                                       230    285    230  230   220                                    Conductor: 24 AWG Copper                                                       Screw Speed:                                                                              50 rpm                                                              Line Speed:                                                                               400 fpm                                                             Die Water Bath                                                                 Distance: 2"                                                                   Foam Insulation                                                                Properties:  KYNAR 2800 blend                                                                             KYNAR 7200 blend                                    Wall (×10.sup.-3 in)                                                                  9.5           8                                                   Density (g/cc)                                                                              1.17          1.08                                                Dielectric Strength                                                                         150           320                                                 (v/mil)                                                                        ______________________________________                                    

Similar results are obtained with copolymers of PVDF and hexafluoropropylene and tetrafluoroethylene, respectively.

EXAMPLE 3

The foamable pellets of Example 1 were tumble blended with 5% by weight PMMA (Plexiglass vs 100 pellets, available from Rohm and Hass Co.). The blend was extruded and coated onto 24 AWG copper conductor under the following conditions.

    ______________________________________                                         1 Inch Extruder (24/1: L/D)                                                             Extruder Zone                                                                  %                                                                                                       Cross-                                                                               Forming                                         1    2      3       Gate head  Die                                    ______________________________________                                                  210  230    285     240  240   230                                    Conductor: 24 AWG                                                              Screw Speed:                                                                              50 rpm                                                              Line Speed:                                                                               500 fpm                                                             Die-Water Bath                                                                 Distance: 2"                                                                                Foamable PVDF Foamable PVDF                                       Foam Insulation                                                                             of Example 1  of Example 1                                        Properties:  Control       with 5% PMMA                                        Wall (×10.sup.-3 in)                                                                  6             7                                                   Density (g/cc)                                                                              1.01          1.10                                                Dielectric Strength                                                                         283           486                                                 (v/mil)                                                                        ______________________________________                                    

The process conditions were run in a manner which gave the unmodified PVDF foam a dielectric strength less than 2 kv in order to emphasize the effect of PMMA, which in this example, increases the dielectric strength well above the 2 kv requirement.

EXAMPLE 4

A foamed PVDF Jacket Insulation, suitable for encasing a group of insulated primary wire, was prepared from a foamable PVDF prepared according to Example 1.

The extruder conditions were as follows:

    ______________________________________                                         1 inch extruder, cable crosshead - pressure type                                        Temperature Profile                                                                                             Die                                           B1    B2     B3     Gate Crosshead                                                                              Tip                                  ______________________________________                                                  200° C.                                                                       230    280    230  230     300                                  Extruder Speed:                                                                           46 rpm                                                              Line Speed:                                                                               20 fpm                                                              Gate Pressure:                                                                            4000 psi                                                            ______________________________________                                    

The foamed jacket insulation formed had a density of 0.80 g/cc and a wall thickness of 65 mils. Such a jacket has the advantages of greater flexibility, lower weight, and lower material cost over conventional jackets.

Similar results are obtainable with polymers consisting essentially of a homopolymer of polyvinylidene fluoride, copolymers or terpolymers containing a major portion of vinylidene fluoride with at least one copolymerizable monomer selected from the group consisting essentially of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene and tetrafluoroethylene and mixtures of the homopolymer, copolymer, and terpolymer.

Other suitable dispersing aids include: dialkyl phthalate, dibutoxyethyl phthalate, diisononyl phthalate, ditridecyl phthalate, dicyclohexyl phthalate, diphenyl akyl adipate, methyl phthalyl ethyl glycolate, and diethylene glycol monobutyl ether.

Similar results are achieved with other nulceating agents, including: calcium carbonate, magnesium oxide, lead oxide, barium oxide, titanium oxide, antimony oxide, magnesium carbonate, lead carbonate, zinc carbonate, barium carbonate, carbon black, graphite, alumina, calcium silicate, molybdenum disulfide, magnesium hydroxide, aluminosilicate, calcium metasilicate, and calcium sulfate.

Other suitable blowing agents include those compounds having the formula ROOC--HN--NH--COOR' wherein at least one of R and R' is selected from the group consisting of secondary and tertiary alkyl groups having from 3 to 5 carbon atoms and the other of R and R' is selected from the group consisting of straight-chain and branched-chain alkyl groups having from 1 to 8 carbon atoms, cycloalkyl radicals having from 5 to 8 carbon atoms, aryl radicals having from 6 to 10 carbon atoms, and alkaryl and aralkyl radicals having from 7 to 10 carbon atoms. 

I claim:
 1. A method for producing a foamable polyvinylidene fluoride material suitable for forming foamed electrical wire insulation having a dielectic constant of less than about 5.0, comprising:(a) blending in an intensive powder blender a mixture of(i) a polymer selected from the group of polymers consisting essentially of a homopolymer of polyvinylidene fluoride, copolymers or terpolymers containing a major portion of vinylidene fluoride with at least one copolymerizable monomer selected from the group consisting essentially of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene and tetrafluoroethylene and mixtures of the homopolymer, copolymer, and terpolymer; (ii) 0.05 to 5.0 percent by weight of the polymer of (i) of a nucleating agent of discreet, insoluble particles having a size distribution whose average length in the longest dimension of less than about 5 microns; (iii) 0.05 to 5.0 percent by weight of the polymer of (i) of a blowing agent having an activation temperature that is above the melting point of said polymer but below the decomposition temperature of said polymer; (iv) 0.05 to 5.0 percent by weight of the polymer of (i) of a dispersing aid selected from the group consisting of dialkyl phthalate, dibutoxyethyl phthalate, diisononyl phthalate, ditridecyl phthalate, dicyclohexyl phthalate, diphenyl alkyl adipate, methyl phthalyl ethyl glycolate, and diethylene glycol monobutyl ether; to provide a substantially homogeneous powdered mixture; (b) melt compounding the substantially homogeneous powdered mixture of (a) with high shear melt compounding means while maintaining the temperature of the mixture below the activation temperature of the blowing agent to provide a substantially homogeneous melted blend of the mixture of (a); and then (c) extruding and quenching the melted mixture of (b) to provide the foamable polyvinylidene fluoride material.
 2. The method as defined in claim 1 wherein in (a)(i) the polymer includes at least 70 mole percent of polyvinylidene fluoride monomer units.
 3. The method as defined in claim 1 wherein in (a)(ii) the size distribution of average lengths of the nucleating agent is less than about 2 microns length in the longest dimension.
 4. The method as defined in claim 1 wherein in (a)(ii) said nucleating agent is selected from the group consisting essentially of calcium carbonate, magnesium oxide, lead oxide, barium oxide, titanium oxide, antimony oxide, magnesium carbonate, lead carbonate, zinc carbonate, barium carbonate, carbon black, graphite, alumina, calcium silicate, molybdenum disulfide, magnesium hydroxide, aluminosilicate, calcium metasilicate, and calcium sulfate.
 5. The method as defined in claim 1 wherein the nucleating agent is calcium carbonate.
 6. The method as defined in claim 1 wherein said blowing agent in (a)(iii) is present in an amount of less than 3.0 percent by weight of the polymer.
 7. The method as defined in claim 1 wherein the blowing agent in (a)(iii) has an activation temperature greater than 220° C.
 8. The method as defined in claim 1 wherein the blowing agent of (a)(iii) is diisopropyl hydrazodicarboxylate.
 9. The method as defined in claim 1 wherein the dispersing aid of (a)(iv) is dibutyl phthalate or dioctyl phthalate.
 10. The method as defined in claim 1 wherein the blowing agent in (a)(iii) is a compound having the formula ROOC--HN--NH--COOR' wherein at least one of R and R' is selected from the group consisting of secondary and tertiary alkyl groups having from 3 to 5 carbon atoms and the other of R and R' is selected from the group consisting of straight-chain and branched-chain alkyl groups having from 1 to 8 carbon atoms, cycloalkyl radicals having from 5 to 8 carbon atoms, aryl radicals having from 6 to 10 carbon atoms, and alkaryl and aralkyl radicals having from 7 to 10 carbon atoms.
 11. The method as defined in claim 1 wherein in (a) the nucleating agent of (a)(ii) and the dispersing aid of (a)(iv) are pre-blended prior to forming the mixture of (a).
 12. The method as defined in claim 1 wherein in (b) twin screw compounding means are used for melt compounding the substantially homogeneous powdered mixture of (a).
 13. The method as defined in claim 1 wherein after step (c) the foamable polyvinylidene fluoride material is then chopped to form pellets.
 14. The method as defined in claim 1 wherein the nucleating agent of (a)(ii) is calcium carbonate, the blowing agent of (a)(iii) is diisopropyl hydrazodicarboxylate, and the dispersing aid of (a)(iv) is dibutyl phthalate.
 15. The method as defined in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 wherein after step (c) the material is heated under sufficient pressure to prevent foaming to a temperature of at least the activation temperature of the blowing agent and then releasing the pressure to provide foamed material.
 16. The method as defined in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 wherein after step (c) the foamable material is heated to a temperature of at least the activation temperature of the blowing agent under sufficient pressure to prevent foaming in an extruder and the heated material is extruded through an electrical wire coating die to provide an insulated wire having an electrical insulating sheath of foamed material on the wire. 